1. Field of the Invention
The invention relates in general to the field of digital systems, and, more particularly, to reducing the power requirements of digital systems. Specifically, the invention relates to a method and apparatus for reducing the power consumed by a bus.
2. Description of the Related Art
With the growing complexity of modem computer systems, designers are constantly seeking more efficient methods to reduce power consumption. Modem computer systems may contain several microprocessors, microcontrollers, and other digital devices connected to each other by a bus. The bus transports data among the microprocessors and other components, and is composed of a number of wire-like connections that function as information-transfer lines, which may be located on a "motherboard" (i.e., main printed circuit board).
A "bus node" or "node," as used in the art, comprises a set of functional entities (e.g., a memory, processor, etc.) which share a common interface to a bus (e.g., the IEEE 1394-1995. serial bus). FIG. 1A illustrates a nodal configuration for a 1394 "node" 100, which consists of a 1394 bus interface 101 together with one or more "units" (e.g., disk drives, modems, computers, etc.) 102, which share the node's common bus interface 101. One skilled in the art would appreciate that the node 100 may also include other types of logic circuits that have not been shown. FIG. 1A also indicates that the bus interface 101 consists of two subsections, one of which is a "physical layer" or "PHY" 103, which provides an electrical bus interface. The physical layer 103 includes one or more bus "ports" 104, which are connection points for bus cable segments 105. Collectively, the bus cable segments 105 constitute a 1394 bus (not shown).
The other layer within the bus interface circuit 101 is a "link layer" or "link" 106, which provides a packet interface between the physical layer 103 and the node's unit(s) 102. The link 106 layer may be used to analyze the data within a received request to determine if one of its local units is the targeted (i.e., destination) unit. If so, the link layer 106 would forward the request to the targeted unit. In any event, the physical layer 103, retransmits the signal on the bus via its remaining ports. A 1394 bus consists of a collection of port-to-port connections between physical layers of several nodes, with each node providing internal signal routing between the ports on that physical layer.
FIG. 1B illustrates a notebook computer 110 with a hard disk drive 111 connected by a cable 112. The hard disk drive 111 is connected to a printer 113 by a cable 114, and the printer 113 is connected to a scanner 115 by a cable 116. The notebook 110 is also connected to a Compact Disk or DVD drive 120 by a cable and to a digital camera 125 by a cable 126. The cables 106, 111, 116, 121, 126 are cables in a bus (e.g., IEEE Std 1394-1995) that has point-to-point connections, which allows these devices to be connected to one another in any desired configuration. A 1394 bus, for example, may also be used to connect internal devices to a platform (e.g., connecting an internal primary hard disk to the notebook computer 110).
Point-to-point connections enable communication to occur by having intermediate devices forward information to the desired receiving device. For example, if a picture was scanned into the scanner 115 to be stored on the CD-ROM 120, the scanner would forward the data to the printer 113 (as well as to any other device which might be attached to another port on the scanner). The printer 113 would forward the data to the hard disk drive 111, which would forward the data to the notebook 110 and then to the CD-ROM 120. One skilled in the art will appreciate that even though the data was not directed to the printer 113, hard disk drive 111, or notebook 111, each plays a role in making sure that the data arrives at the intended destination.
In FIG. 2A, a node 200 includes three units 205-207 connected to a link logic circuit 210 by the cables 208. The physical layer 215 includes three ports 216-218, which enable the node 200 to be connected to as many as three other nodes. One skilled in the art will appreciate that the other physical layers illustrated include more or fewer ports than the three illustrated here. FIG. 2B illustrates a connection of the node 200 to nodes 220, 230, and 240. Specifically, the ports 216, 218 are connected to ports 221 and 231 within the nodes 220 and 230, respectively. The port 217 is coupled to a port 243 of the node 240; the port 251 of node 250 is also connected to port 241 of node 240.
As previously mentioned, each of the nodes 220, 230, 240, 250 contain a link layer and a physical layer. For example, the node 250 includes the unit devices 255-257, the link logic circuit 252, and the physical layer 254. Though most 1394 nodes include at least one unit, certain nodes may contain only a physical-layer circuit. Such "naked PHY" nodes are often useful in relaying and electrically redriving bus traffic, even though these nodes do not generate or accept their own bus traffic, except as a signal relay.
Observation of FIG. 2B reveals that there is no single cable that connects one node to all of the other nodes. For example, the node 250 is not directly connected to the nodes 200, 220, 230, thereby illustrating the "point-to-point" nature of this type of bus. If the node 200 desires to send data to the node 250, one of the unit devices 205-207 would send the data to the link logic circuit 210. The link logic circuit 210 would send a signal to the physical-layer circuit 215, which would apply that signal to the bus via all of the ports 216-218. The signal travels to the ports 221, 231, 243 of the physical layer of nodes 220, 230, 240, respectively. The physical layers 224, 234, 244 corresponding to the ports 221, 231, 243 forward the signal to their associated link logic circuits 222, 232, 242. These link logic circuits then determine if the packet is addressed to one of their local unit devices (e.g., unit devices 245-247). If so, then each link circuit passes the packet to that link's targeted unit(s).
If, for example, a packet is received through port 243 of physical layer 244, it is retransmitted through ports 241 and 248. In addition, the incoming packet is sent to link logic circuit 242, which determines, through packet address-matching, whether the incoming packet is directed to one of this node's local units 245-247. If so, the link logic circuit 242 then sends the packet to the targeted unit, as well as producing an acknowledgment packet that is sent through out all of node 240's physical ports. In this way, the acknowledgment is forwarded by the node 240 and thus arrives at the original packet source node (i.e., the node 200).
Because some of the nodes function as forwarding mechanisms during the transmission of data on the bus, and due to the point-to-point nature of the bus, it becomes desirable to have both the physical layer and the link logic circuit powered during transmission. When the IEEE 1394-1995 protocol was developed, it was developed with the condition that the physical layer and the link logic circuit remain powered at all times. Some "cable-powered" 1394 devices acquire power for the link layer and physical layer from the bus. Alternatively, "self-powered" devices power the local link layer and/or physical layer from a resident power source, such as a battery or a local AC supply ("AC brick").
If, for example, node 200 is providing power for the link circuit 210 and the physical layer 215, then those circuits are said to be powered "locally." Such a node may need to remain fully powered in order to properly power the link logic circuit 210 and the physical layer 215, even when the unit(s) on that node are not in use. For example, if the unit 205 is a camcorder that is not presently in use, node 200 may need to remain fully powered in order to properly power physical layer 215 and link circuit 210. It is desirable to provide a mechanism by which a unit that is not in use may be powered down in order to reduce the power consumption of the attendant node. The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.